The present invention relates to injection molding apparatus and procedures and more particularly to a structure and method for enabling and facilitating the transmission of information from injection mold sensors to a control device without the use of cumbersome and expensive analog hard-wired connections. The present invention also provides increased reliability in the feedback control loop as it enables the user to eliminate numerous junctions which can introduce errors into the control system.
Injection molding is typically done in molds which operate at high temperatures and high pressures within the molds. Typical molds include means to heat the molds at numerous points within the mold in order to ensure that the material injected into the mold remains in a molten state until the mold cavity is completely filled and that no voids exist within the cavity (i.e. hot runner system) as is known to those skilled in the art. In addition, as is known to those skilled in the art, it can be desirable to heat a mold, prior to injecting material therein, in order to control the rate at which the material cools and hardens in order to effect the material properties of the molded product (e.g. material strength, etc.)
In order to effect such control, it is necessary to provide a closed-loop feedback system between the controlled device (e.g., a mold heater) and the mold sensor (e.g., a mold temperature sensor), through a controller of some sort which can utilize the information from the mold sensor and control the controlled device in accordance with a predetermined set of instructions. Currently, information from injection mold sensors is transmitted to a controller in analog form via a hard-wired connection which utilizes sensor specific wires which are physically connected to the sensors and the control device through a series of connectors. These wires, used with readily available connectors, create sensor feedback cables. Each cable typically requires two or more wires per sensor located in the mold to transfer an analog signal.
The number of cables required to transfer the information as applied to, for example, temperature in thermal analog form, from the mold to the control device, is dependent on the number of sensor devices located in the mold, but often times exceeds 48 wires for a typical commercial mold configuration. For example, if a mold requires 30 sensor devices, 60 analog sensor wires would typically be required. In addition, each of the sensor wires is typically arranged such that there are 7 thermal junction points between the sensor device and the control device for each sensor wire. Accordingly, in an injection mold such as the one described above, there would be 420 connections created between the sensor devices in the identified mold and the control device for that mold.
Closed-loop feedback systems such as those described above with numerous wires and connectors can create various problems known to those skilled in the art, including: 1) problems associated with bad connections and cold solder joints which may feedback faulty or intermittent data; 2) inaccurate feedback due to temperature variations along the path of the analog feedback cable; 3) the effects of electrical noise on low level analog signals over the span of the feedback cable; 4) numerous problems caused by the sheer volume of cables and wires required, including problems as simple as storage of the wires and cables, and people tripping over cables located on the floor of the injection mold area; and 5) other problems known to those skilled in the art. As the number of sensors in a given mold increases, so too does the number of wires and connections in a conventional system. Thus, as the mold becomes more intricate or sophisticated and control of the operation of the mold becomes more critical, the chance for induced error in a conventional control system similarly increases. Indeed, one practical limitation on the number of temperature sensors which can be effectively employed in injection molding systems results from the limitation on the number of sensor and control wires which a system and system operator can manage.